The present invention relates to a shaped, Raney metal, fixed-bed catalyst activated in an outer layer.
Activated metal catalysts are known in the field of chemical engineering as Raney catalysts. They are used, largely in powder form, for a large number of hydrogenation, dehydrogenation, isomerization and hydration reactions of organic compounds. These powdered catalysts are prepared from an alloy of a catalytically-active metal, also referred to herein as a catalyst metal, with a further alloying component which is soluble in alkalis. Mainly nickel, cobalt, copper or iron are used as catalyst metals. Aluminum is generally used as the alloying component which is soluble in alkalis, but other components may also be used, in particular zinc and silicon or mixtures of these with aluminum.
These so-called Raney alloys are generally prepared by the ingot casting process. In that process a mixture of the catalyst metal and, for example, aluminum are first melted and then cast into ingots. Typical alloy batches on a production scale amount to about 10 to 100 kg per ingot. According to DE-OS 21 59 736 cooling times of up to two hours are obtained. This corresponds to an average rate of cooling of about 0.2 K/s. In contrast to this, rates of cooling of 10.sup.2 to 10.sup.6 K/s are achieved in processes where rapid cooling is applied (for example an atomizing process). The rate of cooling is affected in particular by the particle size and the cooling medium (see Materials Science and Technology, Edited by R. W. Chan, P. Haasen, E. J. Kramer, Vol 15, Processing of Metals and Alloys, 1991, VCH-Verlag Weinheim, pages 57 to 110). A process of this type is used in EP 0 437 788 B1 in order to prepare a Raney alloy powder. In that process the molten alloy at a temperature of 50 to 50.degree. C. above its melting point is atomized and cooled using water and/or a gas.
To prepare a catalyst, the Raney alloy is first finely milled if it has not been produced in the desired powdered form during preparation. Then the aluminum is entirely or partly removed by extraction with alkalis such as, for example, caustic soda solution. This activates the alloy powder. Following extraction of the aluminum the alloy powder has a high specific surface area (BET), between 20 and 100 m.sup.2 /g, and is rich in adsorbed hydrogen. The activated catalyst powder is pyrophoric and is stored under water or organic solvents or is embedded in organic compounds which are solid at room temperature.
Powdered catalysts have the disadvantage that they can be used only in a batch process and, after the catalytic reaction, have to be separated from the reaction medium by costly sedimentation and/or filtration. Therefore a variety of processes for preparing moulded items which lead to activated metal fixed-bed catalysts after extraction of the aluminum have been disclosed. Thus, for example, coarse particulate Raney alloys, i.e. Raney alloys which have been only coarsely milled, are obtainable and these can be activated by treatment with caustic soda solution. Extraction and activation then occurs only in a surface layer the thickness of which can be adjusted by the conditions used during extraction.
A substantial disadvantage of catalysts prepared by these prior methods are the poor mechanical stability of the activated outer layer. Since only this outer layer of the catalyst is catalytically active, abrasion leads to rapid deactivation and renewed activation of deeper lying layers of alloy using caustic soda solution then leads at best to partial reactivation.
Patent application EP 0 648 534 A1 describes shaped, activated Raney metal fixed-bed catalysts and their preparation. These avoid the disadvantages described above, such as e.g. the poor mechanical stability resulting from activating an outer layer. To prepare these catalysts, a mixture of powders of a catalyst alloy and a binder are used, wherein the catalyst alloys each contain at least one catalytically active catalyst metal and an extractable alloying component. The pure catalyst metals or mixtures thereof which do not contain extractable components are used as binder. The use of the binder in an amount of 0.5 to 20 wt. %, with respect to the catalyst alloy, is essential in order to achieve sufficient mechanical stability after activation. After shaping the catalyst alloy and the binder with conventional shaping aids and pore producers, the freshly prepared items which are obtained are calcined at temperatures below 850.degree. C. As a result of sintering processes in the finely divided binder, this produces solid compounds between the individual granules of the catalyst alloy. These compounds, in contrast to the catalyst alloys, are non-extractable or only extractable to a small extent so that a mechanically stable structure is obtained even after activation. However, the added binder has the disadvantage that it is substantially catalytically inactive and thus the number of active centers in the activated layer is reduced. In addition, the absolutely essential use of a binder means that only a restricted range of amounts of pore producers can be used without endangering the strength of the shaped item. For this reason, the bulk density of these catalysts cannot be reduced to a value of less than 1.9 kg/l without incurring loss of strength. This leads to a considerable economic disadvantage when using these catalysts in industrial processes. In particular when using more expensive catalyst alloys, for example cobalt alloys, the high bulk density leads to a high investment per reactor bed, which is, however, partly compensated for by the high activity and long term stability of these catalysts. In certain cases, the high bulk density of the catalyst also requires a mechanically reinforced reactor structure.
An object of the present invention is therefore to provide a shaped, activated metal, fixed-bed catalyst which largely avoids the disadvantages of known fixed-bed catalysts referred to above.